The present invention relates to a creep preventive arrangement for an automatic transmission.
Automatic transmissions usually comprise a torque converter and a planetary change speed mechanism. In such a transmission, when a start-up range, i.e., a "D" range (forward automatic drive range) or a "R" (reverse drive range), a friction element corresponding to the range (i.e., a rear clutch corresponding to the D range and a front clutch corresponding to the R range) is hydraulically operated to cause the planetary change speed mechanism to assume a first forward gear ratio or a reverse drive, and when in this state, the engine speed is elevated, the engine revolution speed is delivered via the torque converter, the friction element and the planetary change speed mechanism to driving wheels of a vehicle.
Since the power delivery path is established when the transmission is set to D or R range even if the engine is idling, a so-called "creep" phenomeona wherein a vehicle tends to creep when the D range or R range is selected when the vehicle is at a standstill. One measure is to set a select lever to "N" (neutral) when the vehicle is at a standstill so as to render the above mentioned friction element inoperable to interrupt power transmission. However, this inevitably requires a troublesome manipulation of the selector lever every time when the vehicle comes to a halt. Besides, substantial shocks are produced whenever the shift is made from N to D or R, thus causing dicomfort of a driver.
Another measure to prevent the vehicle from creeping is to apply a brake. With this measure, a torque converter is kept at a so called "stall" state where an output element (a turbine runner) is held stationary while an input element (a pump impeller) is driven by an engine, imposing load on the engine even when the vehicle is at a standstill. In order to prevent the engine from stalling under this condition, the engine has to increase its idle speed to maintain its idling operation, thus requiring additional fuel. This explains why a vehicle with an automatic transmission is poor in fuel economy as compared to a vehicle with a manual transmission.